The present invention relates to magnetic resonance data acquisition. It finds particular application in conjunction with multi-shot fast-spin echo (FSE) imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with other imaging sequences, such as single shot FSE, multi-shot gradient echo, and three-dimensional EPI.
Conventionally, magnetic resonance imaging includes the sequential pulsing of radio frequency signals and magnetic field gradients across a region to be imaged. A patient is disposed in a region of interest in a substantially uniform magnetic field. In two-dimensional imaging, an RF excitation pulse is applied as a slice select gradient is applied across the field to select a slice or other region of the patient to be imaged. A phase encode gradient is applied along one of the axes of the selected slice to encode material with a selected phase encoding. In each repetition of the pulse sequence, the phase encode gradient is stepped in regular intervals from a negative maximum phase encode gradient through a zero phase encode gradient to a positive maximum phase encode gradient. In three-dimensional volume imaging, a pair of phase encode gradients are applied along the two axes orthogonal to the read direction.
Often, fast spin echo (FSE) acquisitions are employed in order to acquire a complete magnetic resonance data set, i.e. a complete sampling of k-space, more quickly than conventional single echo acquisitions. A FSE imaging sequence is a multi-echo spin-echo sequence where different parts of k-space are recorded by different spin echoes. In FSE sequences, the echo amplitude is modulated as a function of the echo position. In such sequences, the early echoes have the most magnetic resonance signal and the least artifacts as compared with the later echoes. In addition, the ratio of T1 to T2 contrast varies along the echo train. Generally, it is desirable to encode the echoes to place the most desirable echo at the center of k-space and the least desirable echo at the edges of k-space. Spiral and elliptical centric ordered trajectories through k-space have been employed in order to acquire the central portion of k-space first.
Current magnetic resonance scanners organize three-dimensional FSE acquisitions in one of two ways. In one technique, primary views are assigned to echo numbers within the echo train so as to produce the desired contrast, while successive echo trains step through all secondary encodings until all views are acquired. In another technique, the number of secondary encodings is set equal to the echo train length, and each echo train collects all of the secondary encodings for a fixed primary encoding. Reconstruction of image data acquired using these two methods requires large amounts of fast-access memory in proportion to either the image resolution or the echo train length. While the second technique is more memory efficient, it places severe constraints on slice resolution and contrast.
A typical centric ordered acquisition requires enough memory to store the entire acquisition. For example, in a magnetic resonance acquisition having 1024xc3x971024 resolution, a 4-channel phased array RF coil configuration, 50 slices, and 5 repetitions, approximately 1.6 gigabytes of fast-access memory would be required for one frame. The entire five frame acquisition would require 8 gigabytes of memory.
Therefore, a need exists for time and memory-efficient technique for acquiring and reconstructing multi-shot, three-dimensional magnetic resonance data. The present invention contemplates a new and improved data acquisition and reconstruction method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging is provided which includes exciting magnetic resonance in a volume of interest and inducing a train of magnetic resonance echoes, where the echoes generate magnetic resonance echo signals. An ordered list of k-space views to be sampled is computed. Each magnetic resonance echo of the train is phase and frequency encoded into predetermined regions of a three-dimensional k-space in accordance with the ordered view list. Each encoded magnetic resonance echo is sampled in order to create a row of data. A plurality of one-dimensional Fourier transforms are performed along a first direction on each row of data. The transformed rows of data are assigned and stored in one of a plurality of predetermined fast access memory buffers. Upon sampling a complete plane of k-space, a plurality of Fourier transforms are performed along a second direction which is orthogonal to the first direction. The twice transformed data is stored into an intermediate memory media and upon sampling each k-space view, a plurality of Fourier transforms are performed along a third direction which is orthogonal to the first and second directions, rendering a volumetric image representation.
In accordance with a more limited aspect of the present invention, the computing and phase and frequency encoding steps of the present method include ordering the k-space views to be sampled such that magnetic resonance echoes having a desired T1 and T2 contrast are encoded within the center of the three-dimensional k-space.
In accordance with a more limited aspect of the present invention, the computing step includes ordering the k-space views to be sampled such that successive k-space views are within a common plane along the second direction which is orthogonal to the first direction.
In accordance with another aspect of the present invention, in a magnetic resonance imaging apparatus, a main magnetic field is generated through a volume of interest. Radio frequency pulses are transmitted for exciting and inverting magnetic resonance dipoles within the volume of interest to generate a train of magnetic resonance echoes. Gradient magnetic fields are generated to phase and frequency encode the magnetic resonance echoes and a receiver receives and demodulates the magnetic resonance echoes-to produce a series of k-space views. A method of magnetic resonance data acquisition includes receiving at least one inputted scan parameter and dividing a three-dimensional k-space into a plurality of regions, where each region is to be filled by data from one echo of the train of echoes. An optimized data collection command list is computed in response to the at least one inputted scan parameter. The data collection command list includes a plurality of phase encode instructions to fill rows of k-space which lie in common planes. In accordance with the optimized data collection command list, the plurality of phase-encode views are stepped through with each view producing a row of data. Each row of data is Fourier transformed as it is collected and stored in a plurality of fast access memory buffers. Upon collection of one complete plane of k-space data, a second Fourier transform is performed on the data. The twice transformed data is stored in a magnetic media memory. Upon collecting all of the phase encode views on the command list, a third Fourier transform is performed on the data.
In accordance with another aspect of the present invention, a magnetic resonance volume imaging method includes exciting a plurality of multi-echo imaging sequences, each sequence having n echoes, where n is a plural integer. The n echoes have n progressively changing contrast levels. Each echo is read out along a frequency encode axis and is phase encoded along first and second phase encode axes perpendicular to the frequency encode axis and each other. The echoes are phase encoded such that echoes with a preselected level of contrast are encoded closest to a center of k-space and echoes with a contrast furthest from the preselected contrast are phase encoded closest to a periphery of k-space. Echoes with other contrasts are phase-encoded proportionately in between. The echoes are read out to form data lines which are one-dimensionally Fourier transformed. The one-dimensionally transformed data lines are stored until a full plane of data lines is stored. The stored plane of data lines is Fourier transformed in a second dimension. The two-dimensionally transformed planes of data lines are stored until a full volume of data lines is stored. The full volume of data lines is Fourier transformed in a third dimension to create a volumetric image representation.
In accordance with another aspect of the present invention, a magnetic resonance apparatus includes a main magnet for generating a temporally constant magnetic field through an examination region. A radio frequency transmitter excites and inverts magnetic resonance dipoles within the examination region to generate a train of magnetic resonance echoes. Gradient magnetic field coils and a gradient magnetic field pulse amplifier generate at least phase and read magnetic field pulses in orthogonal directions across the examination region. A receiver receives and demodulates the magnetic resonance echoes to produces a series of k-space views. A view order processor computes a view list of phase-encode k-space views to be sampled. A scan controller steps through the view list and controls the gradient pulse amplifier and radio frequency transmitter to generate a plurality of fast spin echo pulse sequences. A plurality of Fourier transform processors perform a plurality of one-dimensional Fourier transforms along three orthogonal directions on the received k-space views. A fast-access memory selectively stores the once Fourier transformed k-space views and a magnetic media memory stores the twice Fourier transformed k-space planes.
In accordance with a more limited aspect of the present invention, the view order processor includes means for optimizing the order of the view list according to a desired T1 and T2 contrast. The view order processor further includes means for optimizing the order of the view list in order to fill rows of k-space which lie in. common planes.
One advantage of the present invention resides in a time-efficient magnetic resonance data acquisition. and reconstruction technique.
Another advantage of the present invention resides in a memory-efficient magnetic resonance data acquisition and reconstruction technique.
Another advantage of the present invention is that is provides better control of contrast with reduced multi-shot artifacts for three-dimensional multiple echo train sequences.
Another advantage of the present invention is that the data acquisition order may be organized to minimize slab edge defects.
Another advantage of the present invention resides in improved image quality.
Still another advantage of the present invention is that it provides faster image reconstruction.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.